In recent years, standards of USB 2.0 and IEEE1394 are widely distributed as high-speed signal transmission interfaces and used in a large number of digital devices such as personal computers and digital cameras. These interfaces adopt the differential transmission method that transmits a differential signal by using a pair of signal lines to realize faster signal transmission than the conventional single end transmission method.
A common mode filter is widely used as a filter to remove noise on a high-speed differential transmission line. The common mode filter has characteristics that the impedance to a differential component of signals transmitted through a pair of signal lines is low and that impedance to a common mode component (common mode noise) is high. Therefore, by inserting the common mode filter into the pair of signal lines, common mode noise can be cut off without substantially attenuating a differential mode signal.
FIG. 20 is a schematic exploded perspective view showing a structure of a conventional surface-mounted common mode filter.
As shown in FIG. 20, a conventional common mode filter 1 includes a thin-film coil layer 2 containing a pair of coil conductors 5, 6 that are mutually electromagnetically coupled and magnetic substrates 3, 4 provided above and below the thin-film coil layer 2 and made of ferrite. Ends of the coil conductors 5, 6 are each connected to external terminal electrodes 7a to 7d and the external terminal electrodes 7a to 7d are formed on side surfaces and upper or lower surfaces of the magnetic substrates 3, 4. The external terminal electrodes 7a to 7d are normally formed by sputtering or plating of the surface of a magnetic substrate.
WO 2006/073029 discloses a terminal electrode structure of a common mode filter. The terminal electrode of the common mode filter has an Ag film formed by applying a conductive paste containing Ag to the surface of a component or by sputtering or vapor deposition and then a metal film of Ni is formed by performing wet type electrolytic plating on the Ag film.
Japanese Patent Application Laid-Open No. 2007-53254 discloses a common mode choke coil having an outer shape of rectangular parallelepiped by successively forming an insulating layer, a coil layer containing a coil conductor, and an external electrode electrically connected to the coil conductor on a silicon substrate by thin-film formation technology. In the common mode choke coil, the external electrode is formed by extending on the upper surface (mounting surface) of the insulating layer. An internal electrode terminal is constituted as an electrode of a multi-layered structure in which a plurality of conductive layers is stacked.
The conventional common mode filter 1 shown in FIG. 20 has a structure in which a thin-film coil layer is sandwiched between two magnetic substrates and thus has not only high magnetic properties and excellent high-frequency properties, but also high mechanical strength. However, the structure of the conventional common mode filter uses upper and lower magnetic substrates made of ferrite and a ferrite substrate is easy to break when thinned too much, making slimming-down of the substrate difficult. Further, the filter is made thicker by two magnetic substrates being stacked so that it has been difficult to provide a lowered chip component. Moreover, a large amount of expensive magnetic materials is used, posing problems of high manufacturing costs and excessive specs of filter performance depending on uses.
In the conventional common mode filter, micro terminal electrodes are formed on the surface of individual chip components by sputtering or the like, posing a problem that it is extremely difficult to form a terminal electrode with high precision. Further, the internal electrode terminal is formed of many stacked conductor layers in a common mode choke coil described in Japanese Patent Application Laid-Open No. 2007-53254 and thus, the probability of a failed electrode being formed is high and a problem of increased manufacturing costs due to an increase in man-hour for the electrode formation is caused.